There is a method of, when deleting file data in an external storage device (for example, an HDD (Hard Disk Drive) or an SSD (Solid State Drive)) on an operating system (OS), deleting only data management information on the OS without actually erasing data from the external storage device. Delete processing is not always performed in the external storage device for every data delete on the OS. The absence of delete processing leads to improved performance of the operation of the external storage device. This method is particularly effective in an HDD of slow access speed.
On the other hand, the data that should have been deleted on the OS is recognized as valid data on the external storage device (this data is called host-invalid and device-valid data). For this reason, the free area on the external storage device where no data exists is smaller than that recognized by the OS. Exhaustion of the free area on the external storage device may pose a serious problem especially in an SSD in which logical addresses and physical addresses do not necessarily have a one-to-one correspondence.